If you can't find a NOS one get one close that's a little larger and clamp it in a vice and file it down. flip it and file the side that was in the vice. They are generally pretty soft.
Make it a little bigger than the keyway in the crank so it stays put. That will save you from looking for it next time you remove the flywheel.
Having said that, the key just locates the flywheel on the crankshaft. If you mark it and line it up properly you don't need it. Just make sure there is no oil at all on the mating surfaces. Putting oil on there is generally what makes the keys shear.
